Tube bundle for shell-and-tube heat exchanger and method of constructing same

ABSTRACT

A tube bundle is provided for a shell-and-tube heat exchanger. The tube bundle includes a plurality of elongated tubes, each of which has an intermediate portion that has a cross section in the form of a flattened circle with at least one axis of symmetry. The ends of the tubes may have a circular cross section, with the diameter of one of the circular ends being less than the other end and also less than the length of a shorter axis of symmetry of the intermediate portion of the tube. This tube construction allows the tube bundle to be assembled by inserting the smaller-diameter end of the tube through the aligned openings in the spaced-apart baffles in the tube bundle. The non-circular cross section of the intermediate portion of the tubes increases the tube-side and shell-side heat transfer coefficients.

BACKGROUND OF THE INVENTION

The present invention relates generally to shell-and-tube type heatexchangers and, more particularly, to the tube bundles used in such heatexchangers and methods for the assembly and use thereof.

Shell-and-tube heat exchangers are used in a wide variety ofapplications to cause heat exchange between fluid streams. In these heatexchangers, a first fluid referred to as the tube-side fluid is directedthrough elongated tubes in a tube bundle that is housed within a shellthat is typically of cylindrical shape. A large number of the tubes areincluded in the tube bundle and they extend in parallel and spaced-apartrelationship to each other. The tubes are fixed at their opposite endsto generally planar header plates that are also known as tube sheets. Asecond fluid, commonly referred to as the shell-side fluid, flows withinthe shell in the open space surrounding the tubes and undergoes heatexchange with the first fluid stream flowing within the tubes.

Shell-and-tube heat exchangers are constructed in different known waysto provide the desired flow arrangement between the tube-side fluid andthe shell-side fluid. For example, in a single-pass tube and asingle-pass shell arrangement, straight tubes are used and the inlet andoutlet nozzles for the tube-side fluid stream are located at oppositeends of the heat exchanger. The inlet and outlet nozzles for theshell-side fluid are likewise located at opposite ends of the heatexchanger. The flow of the fluid streams in such an arrangement can beeither co-current or counter-current. In co-current flow, the inletnozzles for both the tube-side fluid and the shell-side fluid arelocated at the same end of the heat exchanger and the outlet nozzles forthe fluids are located at the opposite end of the heat exchanger. Thetube-side and shell-side fluids then enter at the same end of the heatexchanger, flow along its length, and exit at the opposite end of theheat exchanger. In counter-current flow, the inlet nozzles for thefluids are located at opposite ends of the heat exchanger and the outletnozzles are likewise located at opposite ends of the heat exchanger. Thefluids then enter at opposite ends of the heat exchanger, flow inopposite directions along its length, and exit at opposite ends of theheat exchanger.

In another flow arrangement, U-shaped tubes are used instead of straighttubes and the inlet and outlet nozzles for the tube-side fluid arelocated at the same end of the heat exchanger. The tube-side fluid flowsalong one leg of each U-shaped tube and then reverses direction andflows back along the other leg of the U-shaped tube. The inlet andoutlet nozzles for the shell-side fluid in this arrangement can both belocated at the same end of the heat exchanger as the inlet and outletnozzles for the tube-side fluid, in which case a longitudinal baffle ispositioned between the legs of the U-shaped tubes to create a dividedflow path that allows the shell-side fluid on one side of thelongitudinal baffle to flow in one direction before reversing andflowing in the opposite direction along the other side of thelongitudinal baffle.

Alternatively, the inlet and outlet nozzles for the shell-side fluid canbe located at opposite ends of the shell so that the shell-side fluidflows in only one direction along the length of the heat exchanger.Other multiple-pass shell and multiple-pass tube arrangements are usedconventionally and are further defined in the Standards of the TubularExchanger Manufacturers Association, which is incorporated by referenceherein in its entirety.

In many applications, the tubes in the tube bundle pass through bafflesthat are spaced apart along the longitudinal length of the tube bundleto provide structural support for the tubes and thereby reduce saggingand vibration of the tubes. Each baffle also serve to divert the flow ofthe shell-side fluid so that it is forced to flow across rather thanalong the tubes to achieve better heat transfer with the tube-sidefluid. The baffles are typically in the form of single or doublesegmental cut baffles in which a quadrant or other region of the baffleis open to allow the passage of the shell-side fluid or disc and donutbaffles in which the shell-side fluid flows through the annular regionsurrounding the disc baffles and through the center openings of thedonut baffles.

During initial assembly of the tube bundle, tie rods are typicallywelded to the baffles to form a cage-like structure in which the bafflesare fixed in spaced apart relationship. The rotational orientation ofeach baffle is set so that the holes in the baffles through which thetubes are inserted are in longitudinal alignment. Because the holes areonly slight larger than the tubes to reduce fluid leakage through theholes, the longitudinal alignment of the holes must be within tighttolerances. Once the tubes have been inserted through the holes in thebaffles, the ends of the tubes are affixed to the tube sheet(s) to formthe completed tube bundle.

The tube sheets in conventional shell-and-tube heat exchangers arenormally formed of high-strength metal or metal alloy and have athickness much greater than that of shell to withstand the operatingpressures within the heat exchanger and to compensate for structuralweakness created by the large number of holes that receive the tubes inthe tube sheets. Fabrication of the tube sheets is a time-intensiveprocess as the holes must typically be individually drilled through thethickness of the tube sheets. The drilling operating creates holes thatare circular in cross section, thereby limiting the tubes to thosehaving the same circular cross section. Although greater heat transfercan be achieved when using tubes having an elliptical or othernon-circular cross section, the circular cross section of the holes inthe tube sheets has heretofore prevented the use of tubes with anelliptical, obround, oval or egg-shaped cross section in shell-and-tubeheat exchangers.

SUMMARY OF THE INVENTION

In one aspect, the present invention is directed to a tube bundle foruse in a shell-and-tube heat exchanger. The tube bundle includes aplurality of hollow, elongated tubes extending in parallel andspaced-apart relationship to each other in a preselected pattern, eachof the tubes having a first end for entry of a first fluid for flowwithin the tube along a longitudinal length of the tube and an oppositesecond end for the first fluid to exit the tube and an intermediateportion between the first and second ends. The intermediate portion ofeach tube has a cross section in the form of a flattened circle. In oneembodiment, the flattened circle has one axis of symmetry. In anotherembodiment, the flattened circle has two axes of symmetry, with one axisbeing shorter than the other. The tube bundle also includes a first tubesheet having holes into which the first ends of the tubes extend and aresecured and a plurality of baffles positioned at spaced apart positionsalong the longitudinal length of the tubes for supporting the tubes andguiding flow of a second fluid exteriorly of said tubes. Each of thebaffles has a cutout for passage of fluid and a plurality of openingsthrough which at least some of said tubes are inserted. The cutouts ofadjacent ones of the baffles are rotationally offset around a centerlongitudinal axis of said tube bundle. The intermediate portion of eachtube is at least a majority of the longitudinal length of each tube, atleast 75% of the length of each tube, at least 90% of the length of eachtube, or at least 95% of the length of each tube. In one embodiment, thefirst and second ends of the tubes have a circular cross section withthe diameter of the second end being less than that of the first end andbeing the same or less than the length of the shorter axis of symmetryof the intermediate portion of each tube.

In another aspect, the invention is directed to a shell-and-tube heatexchanger in which is positioned a tube bundle as described above.

In a further aspect, the invention is directed to a method of assemblingthe tube sheet described above. The assembly method includes the stepsof providing the first and second ends of the tubes with a circularcross section and providing the second end with a diameter that is lessthan the length of said shorter axis of symmetry of the intermediateportion of the tube. The assembly method further includes spacing thebaffles apart from each other a desired preselected distance with theiropenings in alignment, fixing tie-rods to the baffles to secure thebaffles in their spaced-apart and rotational relationship to each other,inserting the second end of each tube through the aligned openings ineach of the baffles, fixing the first ends of each tube in holes in thefirst tube sheet, and fixing the second ends of each tube in other holesin the first tube sheet or a second tube sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view from one end of a heat exchanger made inaccordance with one embodiment of the present invention, with portionsof a shell of the heat exchanger broken away to show an internal tubebundle;

FIG. 2 is a perspective view of the heat exchanger shown in FIG. 1 takenfrom the opposite end;

FIG. 3 is an enlarged fragmentary perspective view of the heat exchangershowing an end portion of the tube bundle;

FIG. 4 is an enlarged fragmentary perspective view illustrating theprocess of assembling the tube bundle by inserting the individual tubesthrough the openings in the baffles;

FIG. 5 is a side elevation view of the heat exchanger taken in verticalsection;

FIG. 6 is an enlarged fragmentary end elevation view taken in verticalsection along line 6-6 in FIG. 5 to show the tubes and tie-rod insertedthrough a portion of one of the baffles;

FIG. 7 is a fragmentary end elevation view of one of the tubes;

FIG. 8 is an elevation view taken from one end of the tube shown in FIG.7;

FIG. 9 is an elevation view taken from the opposite end of the tube; and

FIG. 10 is an illustration of the variables used in the calculation ofthe elliptical cross section of a portion of the tube.

DETAILED DESCRIPTION

Turning now to the drawings in greater detail and initially to FIGS.1-5, a heat exchanger constructed in accordance with the presentinvention is represented broadly by the numeral 10. Heat exchanger 10 isa shell-and-tube heat exchanger and includes an elongated shell 12having a front end 14, an opposed end 16, and an open interior volume18. The shell 12 is of a generally cylindrical configuration, althoughother shapes can be used. The shell 12 is formed of a metal, polymer orother material that is generally inert to the fluid within the shell 12and is able to withstand the pressures and temperatures within the shell12 during operation of the heat exchanger 10.

An inlet nozzle 20 extends from the shell 12 at the front end 14 forintroducing a shell-side fluid into an interior volume 18 of the shell12. An outlet nozzle 22 extends from the shell 12 for removing theshell-side fluid from the interior volume 18 of the shell 12. In oneembodiment, the outlet nozzle 22 is positioned at the opposite end 16 ofthe shell 12 from the front end 14 at which the inlet nozzle 20 ispositioned. In another embodiment, the outlet nozzle 22 is positioned atthe front end 14 with the inlet nozzle 20 and a longitudinally-extendingbaffle (not shown) is positioned within the interior volume 18 of theshell 12. The longitudinally-extending baffle forces the shell-sidefluid to flow from the inlet nozzle 20 to the opposite end 16 of theshell 12 before reversing direction to flow on the opposite side of thebaffle back to the front end 14 where it exits the interior volume 18 ofthe shell 12 through the outlet nozzle 22. The inlet nozzle 20 and theoutlet nozzle 22 typically extend radially from the shell 12, but theymay extend from the shell 12 in other orientations, such astangentially.

In the illustrated single pass tube-side embodiment, an inlet channel orhead 24 defining an interior plenum 25 and having an inlet nozzle 26 forthe tube-side fluid is positioned to close the open front end 14 of theshell 12. An outlet channel or head 28 defining an interior plenum 29and having an outlet nozzle 30 for the tube-side fluid is positioned toclose the open end 16 of the shell 12. In a two pass tube-sideembodiment, the inlet head 24 and outlet head 28 are both positioned atthe front end 14 of the shell 12 and the other end 16 of the shell isclosed. The inlet nozzle 26 and outlet nozzle 30 extend along thelongitudinal center axis of the shell 12 in the illustrated embodiment,but they may extend in other orientations, such as perpendicularly tothe longitudinal center axis of the shell 12.

A tube bundle 32 is positioned in the open interior volume 18 of theshell 12 and comprises a plurality of hollow, elongated tubes 34 thatextend in a parallel and spaced-apart relationship to each other and arepositioned in a preselected pattern. Each of the tubes 34 has an openfirst end 36 for entry of a tube-side fluid for flow within the tube 34along a longitudinal length of the tube 34 and an opposite open secondend 38 for the first fluid to exit the tube 34. The tubes 34 are formedfrom thermally-conductive, corrosion-resistant materials, such asvarious metals, including copper alloy, stainless steel, carbon steel,non-ferrous copper alloy, Inconel alloys, nickel, Hastelloy alloys, andtitanium.

The tube bundle 32 includes a plurality of plate-like baffles 40positioned at spaced apart positions along the longitudinal length ofthe tubes 34. The baffles 40 function to redirect the flow of theshell-side fluid as it flows exteriorly of the tubes 34. The baffles 40also serve to support and maintain the desired positioning of the tubes34. As best shown in FIG. 4, each of the baffles 40 has individualopenings 42 through which the tubes 34 extend. The openings 42 are sizedslightly larger than the tubes 34 to permit the tubes 34 to belongitudinally inserted through the openings 42 while minimizing theamount of the shell-side fluid that can pass through the openings 42.

The baffles 40 are formed as incomplete discs and are sized so thattheir outer perimeters contact or are closely spaced from the innersurface of the shell 12. When the baffles 40 are positionedperpendicularly to the center longitudinal axis of the shell, thebaffles 40 may be formed as incomplete circular discs. When the baffles40 are inclined from the perpendicular, the baffles 40 may be formed asincomplete elliptical discs. The baffles 40 are referred to asincomplete discs because they each include a cutout 44 that allows forthe passage of the shell-side fluid through the baffle 40. The cutout 44in one embodiment intersects the outer perimeter of the baffle 40. Thecutout 44 may be formed as a sector, segment or other portion of thebaffle 40. In one embodiment, the cutout 44 is a sector having an angleof between 45 and 270 degrees, between 75 and 240 degrees, between 85and 230 degrees, 90 degrees, 135 degrees, or 180 degrees.

The cutouts 44 in adjacent baffles 40 are rotationally or otherwiseoffset from each other about the longitudinal center axis of the shell12 to create the desired flow path of the shell-side fluid as it flowswithin the interior volume 18 of the shell 12 from the inlet nozzle 26to the outlet nozzle 30. In one embodiment, the cutouts 44 in thebaffles 40 are hemispheres and the cutouts 44 in adjacent baffles 40 arerotated 180 degrees from each other to create a sinusoidal flow path forthe shell-side fluid. In another embodiment, the cutouts 44 in thebaffles 40 are quadrants and the cutouts 44 in adjacent baffles 40 arerotated 90 degrees from each other to cause create a spiral flow of theshell-side fluid.

The tube bundle 32 may include tie rods 46 that extend longitudinallythrough and are fixed to the perimeter regions of the baffles 40 tosecure the baffles 40 at the desired longitudinal spacing and rotationalorientation. The number of tie rods 46 can be varied as needed. In oneembodiment, between four and twenty-four tie rods 46 are evenly spacedabout the perimeter of the baffles 40.

The tube bundle 32 includes at least one tube sheet 48 that ispositioned at the front end 14 of the shell 12 and separates the openinterior volume 18 of the shell 12 from the interior plenum 25 of theinlet head 24. The tube sheet 48 is normally disc-shaped with aperimeter that seals against the inner surface of the shell 12 in aconventional fashion. As best shown in FIG. 5, the tube sheet 48includes a plurality of holes 49 that extend completely through thethickness of the tube sheet 48 between its opposing faces. The firstends 36 of the tubes 34 are inserted into and secured within the holes49 of the tube sheet 48. If the tubes 34 are U-shaped, the second ends38 of the tubes 34 are inserted into and secured within other holes 49of the tube sheet 48. In the illustrated embodiment in which the tubes34 are straight, a second tube sheet 50 is positioned at the oppositeend 16 of the shell 12 and separates the open interior volume 18 of theshell 12 from the interior plenum 29 of the outlet head 28. The secondends 38 of the tubes 34 are inserted into and secured within the holes49 that extend through the second tube sheet 50. As can best be seen inFIG. 3, in the illustrated embodiment, the first and second ends 36 and38 of the tubes 34 are received within sleeves 51 fitted into the holes49 in the tube sheets 48 and 50.

The tube sheets 48 and 50 must withstand the operating pressures withinthe heat exchanger 10. Because the presence of the holes 49significantly reduces the strength of the tube sheets 48 and 50, thetube sheets 48 and 50 are formed from high-strength material with athickness that is a multiple of that of the shell 12. In one embodiment,each of the tube sheets 48 and 50 is formed of a high-strength metal ormetal alloy and has a thickness of between two and ten inches. Becauseof the hardness and thickness of the material used for the tube sheets48 and 50, the holes 49 in one embodiment of the tube sheets 48 and 50are circular in cross section and are formed in a drilling operation.

In accordance with the present invention, an intermediate portion 52 ofeach tube 34 between the first and second ends 36 and 38 of the tube 34has a cross section in the form of a flattened circle. In oneembodiment, the flattened circle has two axes of symmetry that extend atright angles to each other, with one axis being shorter than the other,to form geometric shapes such as elliptical or obround for the crosssection of the tube 34. In another embodiment, the flattened circle hasonly one axis of symmetry to form a geometric shape such as oval oregg-shaped for the cross section of the tube 34. When the cross sectionof the tube 34 has two axes of symmetry, such as shown in FIG. 10, thetube 34 has a longer axis 54 of a preselected length at the widest partof the cross section of the tube 34 and a perpendicular shorter axis 56at the narrowest part of the cross section of the tube 34. In theembodiment shown in FIG. 10, the cross section of the tube 34 iselliptical and the radii r₁ and r₂ are co-linear at points P₁ and P₂ oftangency of the two curves. At the limits of its domain, radius r₁intersects the longer elliptical axis “a” at focal points Q₁ and Q₂,which is also the center point for the radius r₂. The ratio of theshorter elliptical axis “b” to the longer elliptical axis “a” is in therange of 0.22<b/a<0.92.

The intermediate portion 52 of each tube 34 comprises at least amajority of the longitudinal length of each tube 26, at least 75% of thelength of each tube 34, at least 90% of the length of each tube 34, orat least 95% of the length of each tube 34. As can best be seen in FIGS.7-10, in one embodiment, the first end 36 of each tube 34 is of acircular cross section with a diameter that is greater than that of thesecond end 36. The diameter of the first end 36 of each tube 34 is alsogreater than that of the length of the minor axis 56 (FIG. 10) of thecross section of the intermediate portion 52 of the tube 34 and is lessthan the length of the longer axis 54 (FIG. 10) of each tube 34. Thesecond end 38 of each tube 34 also has a circular cross section with adiameter that is the same as or slightly less than the length of theminor axis 56 (FIG. 10) of the cross section of the intermediate portion52 of the tube 34.

Each tube 34 can be fabricated from stock having a circular crosssection with a diameter that is the same as that of the first end 36.The tube 34 is then flattened by one or more series of rollers to formthe desired geometric shape for the cross section of the intermediateportion 52. The second end 38 is then formed by coning tools to achievea circular cross section with a smaller diameter. As can be seen in FIG.7, each tube 34 includes a transition segment 58 between the first end36 and the intermediate portion 52 and a similar transition segment 60between the second end 38 and intermediate portion 52. The tubes 34 mayeither be bare or they may have extended or enhanced internal and/orexternal surfaces. In one embodiment, the extended surfaces are fins(not shown) that extend longitudinally along the tubes 34.

In one embodiment, the openings 42 in the baffles 40 have the sameelliptical, obround, oval, egg-shaped or other geometric shape as thecross-sectional shape of the intermediate portion 52 of the tubes 34.The openings 42 are slightly larger than the intermediate portion 52 ofthe tubes 34 so that the tubes 34 can be inserted through and maintain atight fit within the openings 42 to reduce the amount of shell-sidefluid that can pass through the openings 42. As one example, theopenings 42 are approximately 0.4 mm larger than the outer dimension ofthe tubes 34. The openings 42 can be arranged with their longer axesextending vertically, horizontally, or an orientation between verticaland horizontal. The openings 42 may each be oriented in the samedirection or they may be independently oriented.

The openings 42 in the baffles 40 are arranged to place the tubes 34 ina preselected pattern, such as a square, rectangular, or triangularpitch. In another embodiment, the openings 42 in the baffles 40 arearranged to place the tubes 34 in a series of concentric circles withthe longer axis 56 of each tube 34 extending tangentially to theassociated circle.

During assembly of the tube bundle 32, the tube sheets 48 and 50 and thebaffles 40 are spaced apart the desired preselected distance from eachother and with their openings 42 in alignment. The tie-rods 46 are thenwelded or otherwise fixed to the baffles 40 to secure the baffles 40 intheir spaced-apart and rotational relationship to each other. One end ofeach tie-rod 46 may be screwed into a threaded hole (not shown) in oneof the tube sheets 48 and 50 and the other end of the tie-rod 46 isfixed to the last baffle 40 at the opposite end of the tube bundle 32.

Once the cage-like structure has been formed by the tube sheets 48 and50, baffles 40, and tie-rods 46, the smaller-diameter, second end 38 ofeach tube 34 is inserted through one of the holes 49 in the tube sheet48 and moved longitudinally the second end 38 passes through one of theopenings 42 in the first baffle 40 at one end of the tube bundle 32. Thetube 34 is then rotated as needed to align the longer and shorter axes54 and 56 of the intermediate portion 52 of the tube 34 with thecorresponding axes of the opening 42 in the end baffle 40. The tube 34is then fed through the aligned openings 42 in the successive baffles 40until the first end 36 of the tube 34 passes through the last baffle 40at the opposite end of the tube bundle 32 and is seated within one ofthe holes 49 in the other tube sheet 50. The remaining tubes 34 areinserted in the same fashion through the holes 49 in the tube sheet 48,the openings 42 in each of the baffles 40, and the holes 49 in the othertube sheet 50. The first and second ends 36 and 38 of the tubes 34 inone embodiment slightly protrude beyond the tube sheets 48 and 50 arethen fixed to the tube sheets 48 and 50, such as by expanding andwelding to the tube sheets 48 and 50. The assembled tube bundle 32 isthen inserted within the shell 12 of the heat exchanger 10 in aconventional fashion.

In use, the shell-side fluid is introduced through the inlet nozzle 20into the interior volume 18 within the shell 12 of the heat exchanger10. The shell-side fluid travels through a sinusoidal path as it travelsalong the length of the shell and navigates through the cutouts 44 inthe baffles 40. The shell-side fluid is then removed from the interiorvolume 18 of the shell 12 through the outlet nozzle 22.

The tube-side fluid is introduced through the inlet nozzle 26 into theinterior plenum 25 of the inlet head 24. The tube-side fluid is thendistributed to the first ends 36 of the tubes 34 and flows along thelength of the tubes 34 before exiting the second ends 38 of the tubes34. The tube-side fluid then enters the interior plenum 29 of the outlethead 28 before exiting the heat exchanger 10 through the outlet nozzle30.

As the shell-side and tube-side fluids travel within the heat exchanger10, heat transfer occurs from one fluid to the other. The non-circularintermediate portion 52 of the tubes 34 in the tube bundle 32 provideshigher tube-side and shell-side heat transfer coefficients thanconventional round tubes because the flattened circle cross section ofthe intermediate portion 52 of the tubes 34 has a greater surface areathan the round cross section of the tubes 34 from which the intermediateportion 52 was formed.

From the foregoing, it will be seen that this invention is one welladapted to attain all the ends and objectives hereinabove set forthtogether with other advantages that are inherent to the structure.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theinvention.

Since many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth or shown in the accompanying drawings is to beinterpreted as illustrative and not in a limiting sense.

What is claimed is:
 1. A tube bundle for a shell-and-tube heatexchanger, said tube bundle comprising: a plurality of hollow, elongatedtubes extending in parallel and spaced-apart relationship to each otherin a preselected pattern, each of said tubes having a first end forentry of a first fluid for flow within the tube along a longitudinallength of the tube and an opposite second end for the first fluid toexit the tube and an intermediate portion between the first and secondends, said intermediate portion of the tube having a cross section inthe form of a flattened circle with at least one axis of symmetry; afirst tube sheet having holes into which the first ends of the tubesextend and are secured; and a plurality of baffles positioned at spacedapart positions along the longitudinal length of the tubes forsupporting the tubes and guiding flow of a second fluid exteriorly ofsaid tubes, each of said baffles having a cutout for passage of fluidand a plurality of openings through which at least some of said tubesare inserted, wherein said cutouts of adjacent ones of said baffles arerotationally offset around a center longitudinal axis of said tubebundle.
 2. The tube bundle of claim 1, wherein said intermediate portionof the tube with said cross section in the form of a flattened circle isa majority of the longitudinal length of each tube.
 3. The tube bundleof claim 1, wherein said intermediate portion of the tube with saidcross section in the form of a flattened circle is at least 75% of thelongitudinal length of each tube.
 4. The tube bundle of claim 1, whereinsaid intermediate portion of the tube with said cross section in theform of a flattened circle is at least 90% of the longitudinal length ofeach tube.
 5. The tube bundle of claim 1, wherein said intermediateportion of the tube with said cross section in the form of a flattenedcircle is at least 95% of the longitudinal length of each tube.
 6. Thetube bundle of claim 1, wherein each of said baffles is positioned in aplane generally perpendicular to the center longitudinal axis of saidtube bundle.
 7. The tube bundle of claim 1, wherein each of said bafflesis a partial disc and said cutout intersects a periphery of the disc. 8.The tube bundle of claim 1, wherein said first tube sheet has additionalopenings into which the second ends of the tubes extend and are secured.9. The tube bundle of claim 1, including a second tube sheet havingopenings into which the second ends of the tubes extend and are secured.10. The tube bundle of claim 1, wherein said openings in said bafflesare shaped to conform to the cross section of the intermediate portionsof the tubes.
 11. The tube bundle of claim 1, wherein said cutout is asector or segment of the baffle.
 12. The tube bundle of claim 1, whereinsaid preselected pattern is a square or triangular pitch.
 13. The tubebundle of claim 1, wherein said preselected pattern is a series ofconcentric circles.
 14. The tube bundle of claim 13, wherein in eachtube the at least one axis of symmetry of the intermediate portion ofthe tube lies along a tangent of one of said concentric circles.
 15. Thetube bundle of claim 1, wherein the cross section of the intermediateportion of each tube has two axes of symmetry with one axis beingshorter than the other and wherein said first and second ends of thetubes are circular in cross section and one of said first and secondends has a diameter that is less than the length of said shorter axis ofsymmetry.
 16. The tube bundle of claim 1, wherein each of said tubes hassaid cross section in the form of a flattened circle along its entirelongitudinal length.
 17. The tube bundle of claim 1, wherein saidflattened circle is in the form of an elliptical or obround geometricshape.
 18. The tube bundle of claim 1, wherein said flattened circle isin the form of an oval or egg-shaped geometric shape.
 19. Ashell-and-tube heat exchanger comprising: a shell having an interiorvolume in which a tube sheet in accordance with claim 1 is positioned;an inlet nozzle extending from said shell for introducing a shell-sidefluid into the interior volume of the shell; an outlet nozzle extendingfrom the shell for removing a shell-side fluid from the interior volumeof the shell; another inlet nozzle for introducing a tube-side fluidinto the first ends of the tubes within the tube bundle; and anotheroutlet nozzle for removing the tube-side fluid from the second ends ofthe tubes within the tube bundle.
 20. A method of assembling a tubebundle of claim 1, said method comprising the steps of: providing saidfirst and second ends of the tubes with a circular cross section andproviding said second end with a diameter that is less than the lengthof said at least one axis of symmetry of the intermediate portion of thetube; spacing the baffles apart from each other a desired preselecteddistance with their openings in alignment; fixing tie-rods to thebaffles to secure the baffles in their spaced-apart and rotationalrelationship to each other; inserting said second end of each tubethrough the aligned openings in each of the baffles; fixing the firstends of each tube in holes in the first tube sheet; and fixing thesecond ends of each tube in other holes in the first tube sheet or asecond tube sheet.